Secondary batteries are widely used as a power source of portable equipment in recent years. In particular, lithium ion secondary batteries can be small in size and low in weight, have a high energy density, and can be repeatedly charged and discharged. On the basis of such properties, lithium ion secondary batteries are expected to grow in demand. Such lithium ion secondary batteries are utilized in apparatuses such as cellular phones and notebook personal computers, taking advantage of the high energy density. Since the apparatuses to which secondary batteries are applied have been improved in performance, secondary batteries are demanded to be further improved in performance. For example, secondary batteries are demanded to have improved performances such as an ability to maintain their capacity even after charge and discharge are repeated in a high-temperature environment (high-temperature cycle property).
A secondary battery is provided with a separator between a positive electrode and a negative electrode in order to improve its performance. As the separator, there is known a porous membrane obtained by applying a layer of a slurry containing non-conductive particles and a binder onto a substrate to form a layer and then drying the layer. As the slurry for forming the porous membrane, a so-called aqueous slurry prepared by using water as a solvent is becoming widespread in order to, for example, reduce load on the environment. Furthermore, it is known to use a variety of polymers as the binder that is a component of the slurry for forming the porous membrane (Patent Literatures 1 and 2).
Such a slurry can be prepared with the physical properties such as viscosity falling within preferable ranges, whereby favorable application and facilitated manufacture of a high-quality porous membrane can be achieved. Furthermore, when a slurry or constituent materials of a slurry are prepared and preserved prior to application, the slurry or the constituent materials is/are required to have a property of maintaining a stable state for an extended period of time without deterioration such as separation of a solid content. For imparting such a property to a slurry, a cellulose-based polymer such as carboxymethyl cellulose (CMC) is often added to the slurry.